1. Field of the Invention
The subject invention relates to the field of heat exchange and humidity removal and, more specifically, to dryers and drying, including cylinder dryers for paper and cardboard manufacture in which the paper, cardboard or other web to be dried runs between the shell of the drying cylinder and a supporting or covering cloth.
2. Disclosure Statement
In the manufacture of paper, carboard and the like, the efficiency of the corresponding machinery has in the past been progressively increased; a trend which represents also a goal for the future. Infortunately, however, the field of contact drying with steam-heated cylinder dryers has scored the lowest progress, even though that area, in terms of installations, traditionally represents the largest part of paper and cardboard machinery. The prevailing low efficiency is thus particularly significant as it determines the capacity of the entire installation and causes disagreeably high production costs.
A certain increase in drying efficiency has been accomplished by the development of cylinder dryers having a concurrently traveling web of felt or screenlike structure. Drying efficiency was also increased at a smaller scale through installation of strips insice the dryer cylinders for stirring up the laminar condensate film, which forms at the cylinder inside wall at speeds in excess of about 400 meters per minute, and for increasing thereby the heat transfer at the cylinder inside wall. In such arrangement, the transverse humidity profile of the web to be dried may somewhat be corrected over the width of the machine through an altering number of inner strips. The effect of such inner strips is, however, not very high, having its cause in the fact that the heat transfer in the cylinder is not the determining parameter for a limitation of the drying process. Rather, the determining factor is the capacity for the removal to the outside of the humidity being evaporated from the web to be dried. As is well known, drying with cylinders in series drying stations takes place in such a manner that the web to be dried runs over part of the circumference of the drying cylinder in heat-transfer contact relationship, and thereafter in a free span to the next drying cylinder, and that such sequence is repeated from drying cylinder to drying cylinder. A large part of the humidity to be evaporated is removed to the environmental air between the particular drying cylinders.
As long as the web to be dried runs over part of the circumference of each drying cylinder, it is most often pressed against the drying cylinder by the supporting fabric or covering cloth, for an increase of the heat transfer from the drying cylinder to the web to be dried. In the course of the particular circumferential cylinder part, the web is heated and the humidity removed.
The latter, however, is only possible to an extent until the outer border layer of air, which is in a laminar or near-laminar state, is saturated with evaporated humidity. In this manner, the achievable capacity of contact drying is very significantly reduced.
From U.S. Pat. No. 3,504,443 it is known to arrange on the outside of the felt or covering cloth a resilient body pressed thereagainst in order to break the border layer of air. In practice, this imposes considerable wear and tear on the traveling covering cloth. In another embodiment, a doctor blade acts as a knife in cutting off part of the border layer and brings about a thinning thereof. In other words, the border layer laden with humidity is "shaved off." A drawback of that technique is, however, that the air located in the meshes or interstices of the covering cloth or structure is not affected thereby, but continues to remain in the meshes to impede an efficient humidity removal. The cited reference thus proposes a use of auxiliary air jets which require special equipment and consume considerable energy.